ATM is a switching and multiplexing technique designed for transmitting digital information, such as data, video, and voice, at high speed, with low delay, over a telecommunications network. The ATM network includes a number of switching nodes coupled through communication links. In the ATM network, bandwidth capacity is allocated to fixed-sized units named “cells.” The communication links transport the cells from a switching node to another. These communication links can support many virtual connections, also named channels, between the switching nodes. The virtual connections, such as a Virtual Channel Connection (VCC), assure the flow and delivery of information contained in the cells.
Each cell contains a cell header and cell data. The cell header includes information necessary to identify the destination of that cell. The components of the cell header include, among other things, a Virtual Channel Identifier (VCI) and a Virtual Path Identifier (VPI), for collectively identifying an ATM connection for that particular cell, and a Payload Type Identifier (PTI), for indicating whether the cell is sent from one user to another, whether cell data refers to administration or management traffic, and whether congestion is present within the network.
The ATM Forum, which is a user and vendor group establishing ATM standards, has also defined several ATM service categories, used in characterization of a virtual connection. For example, there are 16 quality of service categories, among them categories such as (1) a Constant Bit Rate (CBR), which supports a constant or guaranteed rate to transport services, such as video or voice, as well as circuit emulation, which requires rigorous timing control and performance parameters; (2) a Variable Bit Rate (VBR), real time and non real time, which supports variable bit rate data traffic with average and peak traffic parameters; (3) an Available Bit Rate (ABR), which supports feedback to control the source rate in response to changed characteristics in the network; and (4) an Unspecified Bit Rate (UBR).
FIG. 1a illustrates a prior art ATM network 100, typically including several network nodes 110 connected through single communication links 120. Typically, nodes 110 are located in the middle of the network 100, but may also be located at the edges. The network 100 is a data transmission network with guaranteed bandwidth and quality of service. Users 130 access the network 100 and connect to the nodes 110 via similar links 120. Generally, the illustrated communication links support multiple virtual connections. If one link fails, the cells of information transmitted along the virtual connections supported by that link are re-routed using other links in order to avoid extensive loss of data. However, the process of switching and re-routing is time consuming, typically a few seconds, and some loss of data is inevitable.
Attempts made to design a faster switching system for voice communications, preferably under 50 ms, have resulted in the development of an automatic protection switching (APS) system. As illustrated in FIG. 1b, in the ATM network 100, parallel links 122, 124 connect the nodes 110 and are used to transmit duplicate information between the nodes 110 and to ensure fast and reliable data transmission. Link 122 is called an “active” link, while link 124 is a “stand-by” link. Because the same information is transmitted on both links 122, 124, the switching node 110 located at the receiving end can choose either link to receive the transmitted information. For example, if the active link 122 fails, the stand-by link 124 can deliver the same information to the switching node. This APS configuration can be implemented, for example, with the SONET/SDH standards, and can also be used to transport data packets instead of voice communications.
Although the APS system increases the reliability of ATM networks, the duplicated data sent on both links 122, 124 reduces the bandwidth in half, resulting in a waste of bandwidth.